Power amplifiers of the pulse width modulated type are frequently used to provide modulating inputs to an output electrode of an output stage of a high power AM, RF transmitter. Some of the prior art technologies for supplying a modulating input to an output stage of a high power AM RF amplifier are Class B push-pull amplifiers and Class D pulse width modulation amplifiers. Exemplary of prior art patents disclosing such modulation sources are Hulsey et al., U.S. Pat. Nos. 4,747,161 and 4,776,036, Weaver, 4,896,372, as well as Cummings 4,140,980, Swanson 3,506,920 and Swanson 3,588,744. In these prior art devices, the modulating waveform controls the voltage supplied to the anode, i.e., plate, or output electrode, of a power transmitting tube. Basically, the plate DC power supply voltage of the output tube is varied in response to an analog modulation source.
Typically, the pulse width modulated, Class D amplifiers produce varying output signals by changing the on and off times, i.e., duty cycle and frequency, of a switch connected between a DC power supply and the plate of the output stage. In some instances, the amplitude, as well as duration, of current and voltage fed by the switched supply to the output stage is varied. The variable duration current pulses are coupled to the anode of the output stage via a low pass filter, to substantially reconstruct the original analog input signal to the power amplifier.
To obviate the disadvantages associated with high voltage switching, e.g., 30,000 volts, some power amplifiers use multiple pulse width modulation stages controlled by phase displaced pulses having the same duty cycle; see, e.g., U.S. Pat. Nos. Swanson 4,468,626, Swanson 4,164,714 and Kyrian et al 4,369,409. In these modulators, referred to in the art as polyphase pulse width modulators, the stages are switched at the same frequency, but with relative phase differences. The power fed through the multiple stages is summed.
A disadvantage of the polyphase pulse width modulation arrangement is that usually all of the stages are being switched simultaneously. This results in all of the stages being susceptible to narrow pulse distortion, as described in Hulsey, U.S. Pat. No. 4,776,036. Hence, a problem with the polyphase pulse width modulation source is that the modulating output voltage is in certain instances not an accurate replica of the analog signal supplied to the device. This results in distortion of the modulating waveform, with resulting distortion in the transmitted signal of the AM RF transmitter.
It is, accordingly, an object of the present invention to provide a new and improved power amplifier responsive to an analog signal source, wherein the amplifier derives a relatively faithful replica of the source.
It is an additional object of the present invention to provide a new and improved method of operating a power amplifier responsive to an analog signal source to cause the amplifier to derive a relatively faithful replica of the source.
Another object of the invention is to provide a new and improved high power AM RF transmitter that derives a relatively distortion free output signal.
A further object of the invention is to provide a new and improved very low distortion high power amplifier using multiple stages that are switched between a fully conducting, saturated state and a nonsourced state such that only one of the multiple stages is switching at a time to achieve high efficiency and low switching losses.